As a next generation power semiconductor device material, silicon carbide (which may be hereinafter also denoted as SiC) attracts attention. SiC has about 10 times the breakdown field strength and about three times thermal conductivity of silicon (which may be hereinafter also denoted as Si), and SiC can achieve a power semiconductor device capable of operating at a high temperature with a low loss, which cannot be achieved with a Si power device.
For example, a high voltage power MOSFET has a low ON-resistance and high breakdown voltage, and can achieve fast switching operation. Accordingly, it is widely used as a switching device for a power circuit such as a switching power supply. The device structure of the high voltage power MOSFET has a vertical-type MOSFET structure in which a source electrode, a gate electrode, and a well electrode are formed on a substrate surface, and a drain electrode is formed on a back surface of the substrate. Double Implantation MOSFET (which may be hereinafter also denoted as DIMOSFET) structure in which a channel formation region (well region) and a source region are respectively formed on a substrate surface using ion implantation is an advantageous device structure in which the channel region can be easily formed with high precision, and this is also suitable for parallel operation.
When a DIMOSFET using a SiC substrate is formed, an electrode for connecting this device to an electrical circuit and the like is desired to be in ohmic contact. However, a generally used hexagonal single-crystal SiC substrate has 4H—SiC structure of which laminating cycle is 4, and an energy band gap thereof is 3.26 eV, i.e., three times the energy band gap of Si. Therefore, it is difficult to form ohmic contact with a metallic electrode.
To solve this problem, a method for reducing the contact resistance has been suggested, in which the crystal structure of a source region is changed into a cubical crystal SiC structure having a lower energy band gap than that of the 4H—SiC structure (which may also be hereinafter referred to as 3C—SiC structure). More specifically, when the source region is formed, the 4H—SiC substrate of the source region is made into amorphous state by ion implantation, and the SiC having 3C—SiC structure is recrystallized with subsequent high-temperature thermal treatment. In this case, an energy band offset between 4H—SiC and 3C—SiC mainly occurs in a conduction band, and Schottky barrier is reduced about 0.9 eV, so that the contact resistance between the n-type SiC and the metallic electrode is reduced.